Flow reversals and deep-water temperature and salinity trends in a Mediterranean Channel (2003-2019)

The Sardinia Channel (Mediterranean Sea) is a wide opening between Tunisia and Sardinia which has a sill at about 1900 m in a narrow deep trench that allows exchanges of the upper part of the deep waters to occur between the Algerian and the Tyrrhenian subbasins. The densest part of Western Mediterranean Deep Water (WMDW), which is trapped in the Algerian subbasin, is thus overflowing the sill when uplifted by recent and even denser WMDW. The less dense WMDW, circulating anticlockwise and alongslope may enter the Sardinia Channel directly following the Algerian slope. There is another water mass exchanged in this region, the Tyrrhenian Deep Water (TDW) which is a mixing product between the WMDW and waters coming from the Eastern Mediterranean.

The monitoring of thermohaline properties and currents has been operated at the sill between 2003 and 2019, to observe the variability of the deep water exchanged between the two adjacent subbasins. The θ time series collected at the sill since July 2003 shows an alternation of WMDW presence (lower θ and S, generally flowing eastward) and TDW presence (“pulses” of higher θ and S, generally flowing westward). Those TDW pulses are generally of short duration (between 1 day and 1 week) and are likely to be due to displacements of the interface between the two deep water masses. Thus, the mooring alternatively sampled WMDW (mainly) and TDW.

The Sardinia Channel trend component shows a continuous warming trend at a rate of 0.0067± 3.47*10^-6 °C year^-1, accounting for a total deep temperature increase of about 0.114 °C from 2003 to 2019. The salinity trend also shows a salinification at a rate of 0.0032 ±  2.02*10^-6 year^-1, with a total deep salinity increase of about 0.054 from 2003 to 2019. The density trend component shows a continuous densification at a rate of 0.0011± 9.60*10^-7  kg m^-3 year^-1, with a total deep density increase of about 0.0187 kg m^-3 from 2003 to 2019.

The ongoing climate change has amplified the scientific interest in time series and their importance is increasingly recognized even at the political level. Nonetheless, due to their high maintenance costs and the difficulty in maintaining them, they are still widely lacking. It is important to stress that an understanding of physical-chemical-biological processes in the oceans requires regular and long-term observations, that enable us to separate real long-term trends in environmental drivers from the natural variability of the system.